This invention relates to fingerprint authentication method, program and device.
The fingerprint is a pattern of distinctive epidermal ridges on the bulbs of the inside of the end joints of fingers and thumbs. The fingerprint has uniqueness and immutability. Even if the fingerprint is damaged, undamaged corium under the damaged fingerprint ensures that the fingerprint is regenerated. Therefore, the fingerprint can be used to identify each individual person with a high accuracy.
A fingerprint authentication device is used in a security system for various machines and systems. For instance, the fingerprint authentication device allows a regular or normal user, who is previously enrolled in a computer or an information service system using a computer network, to login the computer or to receive information services from the information service system through the computer network. The fingerprint authentication device uses a fingerprint image provided by a user who intends to login the computer or to receive the information services from the computer network to judge whether the user is a registrant (or the regular user) or not.
A related fingerprint authentication device essentially comprises:                (1) a fingerprint sensor (or a fingerprint input portion) for receiving the fingerprint image provided by the user into this system;        (2) a feature extracting portion for processing input data to extract features of the fingerprint image which are used for collation;        (3) a database for memorizing enrolled data (or template(s)), which are previously produced, about the regular user(s); and        (4) a collating and judging portion for comparing (features of) the input data with (features of) the enrolled data to judge whether the user is the registrant. Such a fingerprint authentication device is disclosed in Japanese Unexamined Patent Publication No. Hei 4-33065.        
When there is certain degree of similarity between the features of the input data and the features of the enrolled data, the fingerprint authentication device identifies the user who provided the fingerprint image as the enrolled normal user and allows the user to perform a predetermined operation, such as login a computer.
The fingerprint input portion converts a ridge pattern, which is based on minute concavities (or grooves between ridges) and convexities (or ridges) of a surface of an object (i.e. a finger) placed on the fingerprint sensor, into a digital image data (as the input data). In many cases, the fingerprint input portion includes an optical system. As the optical system, it is known that a method obtains a high contrast image by means of total reflection of a prism. For the method, a photo acceptance device such as a CCD receives light emitted by an LED light source and reflected at an inner side of a reflecting surface of the prism. When a finger of the user is put on an outer side of the reflecting surface of the prism, the concavities and the convexities thereof change reflectance of the reflecting surface. Thus, the photo acceptance device produces the digital image data according to the concavities and the convexities of the finger put on the prism.
In another optical system, light is applied to the finger put on the fingerprint sensor from light sources placed in the vicinity of the fingerprint sensor. The light is scattered at the inside of the finger and a part thereof reaches the fingerprint sensor. The light received by the fingerprint sensor includes difference of strength according to the ridges and grooves of the fingerprint. The fingerprint sensor produces the digital image data on the basis of the strength of the light. Such an optical system is disclosed in Japanese Patent Publication No. 3150126 (P3150126).
The feature extracting portion extracts the features of the fingerprint image represented by the digital image data to collate with features represented by the enrolled data. The enrolled data is also produced by the feature extracting portion previously and memorized in the database.
The collating and judging portion calculates degree of similarity between the features based on the input data and the features based on the enrolled data. Furthermore, the collating and judging portion compares the calculated degree of the similarity with a threshold value. When the calculated degree is larger than or equal to the threshold value, the collating and judging portion judges that the input data and the enrolled data have a common origin. That is, an authentication request is accepted by the fingerprint authentication device. On the other hand, when the calculated degree is smaller than the threshold value, the collating and judging portion judges that the input data and the enrolled data have different origins. That is, the authentication request is rejected by the fingerprint authentication device.
Incidentally, the reason why the authentication request is rejected by the fingerprint authentication device is not clear for the user. The fingerprint authentication device does not accept the authentication request not only when the user is unregistered but also when input of the fingerprint is improper. In other words, when the input data is inappropriate for the authentication, the fingerprint authentication device denies the authentication. Accordingly, an informing system is necessary to inform the user that the input of the fingerprint is improper.
The input of the fingerprint considerably depends on physical relationship between the finger of the user and the fingerprint sensor. There are some systems for deciding whether the physical relationship between the finger of the user and the sensor is correct or not. Such a system is disclosed in each of Japanese Unexamined Patent Publication No. TokkaiHei 8-110949, No. TokkaiHei 8-161491, No. TokkaiHei 9-274656, and No. Tokkai2001-266133 (P2001-266133A).
However, propriety regarding to the input of the fingerprint depends on not only the physical relationship between the finger of the user and the fingerprint sensor but also other things. For example, it is necessary to consider (1): whether outside light unrelated to a light source(s) for the fingerprint sensor exceeds tolerance level or not; and (2): whether an object put on the fingerprint sensor has characteristics suitable for being input or not.
The fingerprint authentication device uses minutiae, such as endpoints and branch points of ridges of the fingerprint image. Accordingly, the fingerprint authentication device tends to mistake (1): when the outside light unrelated to the light source(s) for the fingerprint sensor exceeds the tolerance level; (2): when the object put on the fingerprint sensor does not have the characteristics suitable for being input; and/or (3): when the object is properly put on the fingerprint sensor. Accordingly, the fingerprint authentication device must judges whether the input fingerprint image is proper or not concerning the above mentioned (1), (2) and (3) to obtain a higher accuracy of authentication. If necessary, the fingerprint authentication device must reflect the result of judgment concerning the above mentioned (1), (2) and (3) on the authentication. Alternatively, the fingerprint authentication device must request the user to put the finger on the fingerprint sensor again according to the result of judgment concerning the above mentioned (1), (2) and (3).
To perform the judgment concerning the above mentioned (1), (2) and (3), the fingerprint authentication device is possible to include additional sensors. For instance, an optical power sensor, a conductive sensor and a pressure sensor can be used for the above mentioned (1), (2) and (3), respectively. That is, the optical power sensor can be used to detect background light. The conductive sensor can be used to electric resistance of the object put on the fingerprint sensor. The pressure sensor can be used to pressure provided from the object on the fingerprint sensor. The fingerprint authentication device processes output signals from the additional sensors in parallel to process the input date from the fingerprint sensor. The fingerprint authentication device performs the authentication in a comprehensive manner using processing results of both the output signals from the additional sensors and the input data from the fingerprint sensor.
However, the method using additional sensors has problems that special hardware for the additional sensors is necessary thereby cost increases. Furthermore, the method needs a long time for performing the authentication thereby it lacks convenience.